Several approaches will be used to define factors that regulate hemopoietic cell growth and differentiation in normal and malignant conditions. We shall examine mechanisms underlying the anemia and the frequent leukocytosis of animals with transplantable tumors, with specific regard to the effects of excessive levels of prostaglandins of the E series and the role of activated T suppressor cells in hemopoiesis in vivo. We shall also investigate the effects of hemin on cell growth and differentiation in order to examine the mechanism by which hemin induces gene activation in erythroid and possibly non-erythroid hemopoietic cells and to extend our recent observations that hemin interacts with anthracycline drugs, perhaps at the DNA level, in different ways, enhancing the induction of differentiation of the K-562 leukemic cell line and preventing leukemic cell destruction by such potent anthracyclines as adriamycin.